The present invention concerns an underwater pipeline for transferring petroleum products and more particularly a pipeline of the above kind comprising at least one flow line for the above products and a protective carrier pipe around said flow line. More particularly, the present invention concerns a pipeline of the above kind designed to be laid on -or near the sea floor at great depths, i.e. at depths of several hundred metres or more.
At these depths underwater wells usually produce liquid hydrocarbons, gaseous hydrocarbons and water simultaneously. At the low temperatures encountered at these great depths the multiphase mixture causes the formation of gas hydrates and paraffins that can block the flow lines in which it flows.
One solution to the problem of preventing blocking of the flow lines is to cover them with an external coating that adheres to the flow lines and is made from a thermally insulative material which also has the mechanical strength needed to withstand the high hydrostatic pressure encountered at great depths. In particular, composite products based on an epoxy resin, polyurethane or polypropylene, for example, are used for this coating. Such products are manufactured and sold by companies including Isotub (France), Balmoral Webco Pipeline Systems (Great Britain) and Bredero Price (USA), for example. By adjusting their composition it is possible to vary the density (and therefore the buoyancy), the mechanical strength, the coefficient of heat transfer, the thermal conductivity, for example. The high mechanical strength of the coating needed at great depths is accompanied by a great increase in the density of the material used, this increase in density having an adverse effect on its thermal insulation properties. The thickness of the coating must then be further increased to obtain the required thermal insulation, which renders this solution excessively costly at great depths. Also, the resistance to abrasion of such a coating is insufficient to allow flow lines installation by towing them along the sea floor.
Another technique known in itself for protecting underwater flow lines at great depths is to surround the flow lines with a conventional tubular protective carrier pipe capable of resisting the hydrostatic pressure. The flow lines protected by such a carrier pipe may be then installed by towing them into place. The carrier pipe can contain a plurality of flow lines, each having a thin thermal insulation coating of low density (polyurethane foam, polyethylene foam, glass wool, mineral wool, etc). At very great depths the steel carrier pipe cannot resist the hydrostatic pressure unless its thickness is increased out of all proportion, which increases the weight per linear metre to the detriment of buoyancy, which is necessary for towing it into place. In this latter case, which is well known in the art, the internal space of the sleeve between the coated flow line(s) and the sleeve itself is filled with an inert gas, for example nitrogen. The pressure of nitrogen in the sleeve must then be maintained throughout the service life of the flow lines, which can be 20 years or more. This constraint is costly because the initial pressurisation is costly and maintenance is difficult because the nitrogen slowly diffuses through the welds in the carrier pipe. Moreover, the pressure must be established during the construction of the pipeline on land, for example on a beach. The high pressure required can then cause a dangerous explosion of the carrier pipe. It is therefore necessary to increase the thickness of the carrier pipe which is detrimental to the buoyancy of the assembly, which is indispensable to towing it into place, as already mentioned.